Voltage controlled amplifiers have been known in the art of audio signal processing for many years. Such amplifiers are used to suppress amplifier gain when the amplitude of an audio input signal exceeds a particular level. Such compressor limiters seek to attenuate high level input signals to maintain the same within a suitable range for the receiving output apparatus such as speakers or the like. This attenuation of the input signal by adjustment of amplifier gain is to reduce distortion which would otherwise occur at the output apparatus if the audio signal received thereat is of excessive amplitude. The prior art has taught that amplifier gain is suppressed anytime that the input signal exceeds a set threshold level.
In the prior art illustration of FIG. 1, there is shown a compressor limiter 10 of the feed forward type. Here, a voltage controlled amplifier 12 receives an input signal and amplifies it according to an adjustable gain to provide an output signal as shown. The adjustable or variable gain is controlled by a rectifier 14, preferably a full wave rectifier, which receives the input signal and passes it to an integrator or other RMS converter-type circuit 16. In essence, the circuit 16 functions as a filter. The output of which is passed to an amplifier 18 which has a threshold associated therewith. This threshold is set by a potentiometer, voltage divider, or the like 19. The amplifier 18 thus operates as a threshold comparator, providing an output signal corresponding to the relationship between the threshold set at 19 and the output of the filter or RMS converter 16. The output of the threshold comparator 18 is then fed to the voltage controlled amplifier 12 to appropriately adjust gain.
Another configuration of the prior art is illustrated in FIG. 2. Here, a compressor limiter 20, again of the feed forward type, is shown utilizing a voltage controlled amplifier 22 for receiving the input signal, appropriately amplifying it, and providing it to an output point. Control of the gain of the amplifier 22 is achieved in somewhat the same manner as that of FIG. 1. Here, a rectifier 23 receives the input signal and supplies it to a comparator 24 having a threshold at the positive input thereof, which is shown as being adjustable by means of a potentiometer 28. The output of the comparator 24 is then integrated or filtered as at 26 prior to application to the amplifier 22. In this situation, the gain of the amplifier 22 is adjusted according to the filtered comparison of the rectified input signal against a set threshold level.
Yet another embodiment of the prior art is depicted in FIG. 3, as the compressor limiter 30 of the feedback type. Again, a voltage controlled amplifier 32 receives an audio input signal, but the gain is now controlled by a feedback circuit comprising elements 34-39. The signal is, of course, rectified as by a full wave rectifier 34, and thence passed to an RMS converter, integrator, or filter 36. The output of the circuit 36 is then applied to a threshold comparator 38, having a threshold set as by a voltage divider or potentiometer 39. The output of the comparator 38 is fed back to the voltage controlled amplifier 32 to appropriately adjust gain.
Known prior art compressor limiters utilize some type of integrator 16,26,36, which of necessity incorporate an integrating or filtering capacitor. Accordingly, with the presence of such a capacitor, the prior art compressor limiters have had associated therewith inherent phase shifts between the input and the control signal. The phase shift of the control signal allows the input signal to pass through the amplifier before the voltage controlled amplifier gain can be adjusted. Further, it is known that audio signals are not sinusoidal but, indeed, vary broadly as to both amplitude and frequency. Accordingly, integration of audio signals is not given to basic integration formulas as with resistor-capacitor circuits where: ##EQU1## The standard integration formula is only theoretically applicable to audio signals since such signals are constantly changing with respect to both frequency and amplitude. As a point of fact, audio signals are of broad ranges of duration. Since integrators are typically designed for particular frequencies, amplitudes, and signal durations, it is virtually impossible to provide a resistor-capacitor integrator which will operate over the requisite broad range of frequencies, amplitudes, and pulse durations. Accordingly, when utilizing a resistor-capacitor type integrator or circuit, errors are introduced to the control voltage applied to the voltage controlled amplifier, as to both phase shift and amplitude. Of course, such errors lead to distortion of the output audio signal.
In previously known compressor limiters, the capacitor in the filter or integrator has had associated therewith a specific release time, such release time being directly associated with the discharge rate of the integrating or filtering capacitor. These capacitors have typically been of a significantly large size such that release times were on the order of 100 milliseconds to 5 seconds. When the release time was shortened as by reduction of the capacitor size, the distortion of the output signal increased. Accordingly, the prior art has been shackled with a selection to be made between short release times and distortion. Further, the release time of the prior art has been initiated anytime that an input signal exceeded the threshold. At such time, the capacitor was charged and the release time was then set by the discharge rate of the capacitor. In effect, the prior art compressor limiters were unaffected by the duration of time that the input signal exceeded the threshold, but only by the fact that the threshold was exceeded.